Gratz on the pull- he is fun and i use him in tag sometimes  but i find he doesnt win on defence and i suspect he wont win on the harder live arena matches as he has obvious counters for people with big rosters.

Gorgid will just oneshot him through the stoneskin and harima is a hard counter as well.

I find him better on attack against bad AI than on defence where people avoid hitting him or use specific champs to counter him

If Mortu is your only damage dealer, the easiest counter is not to use AoE attacks until his stoneskin expires. 

I would only chose Mortu (if I had him at all) if the opponent picks champs that can't avoid AoE attacks, like Krisk or Skullcrown. Otherwise, Mortu just sits in his stoneskin for 2 rounds and does nothing usefull.

For Tag arena he is unquestionably one of the best in the game on offense, because it's very easy to just arrange to only use him against teams that will let you get Peril to proc.

On defense, I still think he has value, simply because he forces the other team to use a specific team against him, which limits their ability to properly field teams against your other teams.

For Live Arena though, I've basically stopped using him. It's just too easy to ignore him.

Couldn't agree more.

He's been terrible for me in Live. But he's still great in Tag and Arena Offense.

I agree, when i have used a mortu i have paired him with a UDK to try and make single targetting him down a bit more tricky.